Our multifaceted investigation on normal and atypical melanocytes concerns the following topics: 1. Characterization of the receptors for MSH on melanoma cells. 2. The mechanism of internalization of the MSH-receptor complex. 3. The mechanism by which cAMP increases the activity of tyrosinase. 4. The cytotoxicity of substances produced by the tyrosing-tyrosinase reaction. 5. The culturing of normal and malignant melanocytes in synthetic medium. 6. The characterization of virus particles in melanomas.